Field of the Invention
The invention relates generally to motors for electric submersible pump (ESP) systems, and more particularly to systems and methods that employ a conductive roller to provide an electrical pathway between the rotor and stator of an ESP motor and thereby prevent an electrical potential from developing between these components.
Related Art
Oil and natural gas are often produced by drilling wells into oil reservoirs and then pumping the oil and gas out of the reservoirs through the wells. If there is insufficient pressure in the well to force these fluids out of the well, it may be necessary to use an artificial lift system in order to extract the fluids from the reservoirs. A typical artificial lift system employs an electric submersible pump which is positioned in a producing zone of the well to pump the fluids out of the well.
An electric submersible pump system includes a pump and a motor which is coupled to the pump and drives the pump. The electric submersible pump system may also include seals, gauge packages and other components. Because they are designed to fit within the borehole of a well, electric submersible pump systems are typically less than ten inches wide, but may be tens of meters long. The motor of an electric submersible pump system may produce hundreds of horsepower.
The motor of the electric submersible pump system is typically an AC induction motor. The motor has a stator that is cylindrical with a coaxial bore. A rotor is coaxially positioned within the bore of the stator. The rotor is coupled to a shaft so that rotation of the rotor turns the shaft. Bearings hold the rotor in position within the bore of the stator and allow the rotor to rotate smoothly within the bore.
During the normal operation of an induction motor, a voltage may develop between the shaft, rotor and stator. The voltage difference between the rotor and stator may cause electrical arcs to occur between these components, as well as and other related components such as rotor bearings. Increased use of Pulse Width Modulation (PWM) type motor controllers has made this problem more common and acute.
The occurrence of electrical arcs between the rotor, stator and other components can cause damage to the components. The electrical arcs may, for example, cause the surfaces of the components to be pitted, and may cause small particles to break away from the surfaces. The oil flowing through the motor may carry these particles to other parts of the motor, causing increased wear on the components and shortening the life of the motor.
In some previous systems, the problem of arcing between the rotor and stator was addressed by attaching an electrically conductive brush to the stator housing. The brush made of a material such as beryllium-copper is positioned to contact the shaft of the motor. Typically, the brush is spring-loaded to maintain contact between the brush and the shaft. As the shaft and rotor rotate within the bore of the stator, the brush maintains contact with the shaft. In this manner, an electrical pathway is maintained between the rotor and stator.
While the brush maintains an electrical pathway between the rotor and stator and consequently prevents arcing between these components, this approach also has some problems. In particular, because the brush is constantly rubbing against the shaft, the friction between these components can cause small particles to be scraped off the contacting surfaces of the components. These particles are then carried by the oil throughout the motor, where they can damage the various components of the motor. Thus, the use of a brush against the shaft to maintain an electrical pathway between the rotor and stator results in one of the same problems that it was intended to alleviate.
It would therefore be desirable to provide means to prevent or reduce arcing between the stator and rotor without creating particles that may be suspended in the oil and may damage the components of motor.